This invention relates to a method and an apparatus for torque ripple reduction in electric motors.
Most motor vehicle steering systems produced today employ some type of power assist steering system to assist the driver in steering the front wheels. In general, power assist steering systems employ a hydraulic pump to provide pressurized fluid to a piston connected to or formed in the steering rack assembly, the pressure being regulated by a valve which is opened or closed by an amount that varies with the torque in the steering column. Thus, as the driver exerts more effort against the steering wheel, the valve is opened to provide more fluid to the piston, thereby assisting the driver in steering the vehicle.
It has been heretofore known to provide a power assist system using an electric motor as the source of motive force as opposed to pressurized hydraulic fluid. This would improve fuel economy and reduce the manufacturing cost of the vehicle. Furthermore, it is suggested that reliability of the system would be improved since many components including the hydraulic pump and fluid lines can be eliminated. However, prior attempts at making an electric power assist steering system have proved unsatisfactory.
Perhaps the most vexing issue with electric power assist steering systems has been torque ripple felt at the hand wheel. Torque ripple is the variation in reaction torque felt by the driver as the hand wheel is turned. Because drivers are so used to the smooth response of the hydraulic power assist steering systems, they have generally reacted unfavorably to electric power assist steering systems due to the presence of torque ripple.
Most torque ripple is caused by the electric motor that provides the power assist motive force. There are two primary sources of torque ripple in the electric motor. The first is known as cogging torque, which is caused by the magnetic attraction between the rotor mounted permanent magnets to the stator. It has been known to skew the rotor magnets and to use more expensive surface-parallel type magnets to reduce the effect of cogging torque, however, the magnetization process and machining process for skewed and for surface parallel magnets is higher, thereby reducing the benefits of the electric power assist steering system. The other primary source of torque ripple is the harmonics content in the line-to-line back-emf due to an imperfect sinusoidal back-emf waveform.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a method of selecting a magnet angle for a permanent magnet electric motor including determining a first magnet angle where cogging torque is minimized, determining a second magnet angle where harmonic content of nth harmonic is minimized, and using the first magnet angle and the second magnet angle, determining an optimal magnet angle for minimizing both cogging torque and nth harmonic.
In another embodiment, a method of reducing torque ripple in an electric motor includes providing a first magnet ring on a rotor positioned about a shaft of a motor, the first magnet ring having magnets each occupying a magnet angle xcex4 on the rotor, providing a second magnet ring on the rotor, the second magnet ring having magnets each occupying a magnet angle xcex4 on the rotor, and shifting the second magnet ring a non-zero number of degrees relative to the first magnet ring wherein ends of each magnet within the second magnet ring are located at different angular positions than ends of each magnet within the first magnet ring relative to a shaft axis of the shaft.
In another embodiment, a motor for reducing torque ripple includes a shaft having a shaft axis, a rotor positioned about the shaft, a first magnet ring positioned on the rotor, the first magnet ring comprising magnets each occupying a magnet angle xcex4 on the rotor, and a second magnet ring positioned on the rotor, the second magnet ring comprising magnets each occupying a magnet angle xcex4 on the rotor, wherein the second magnet ring is shifted a non-zero number of degrees relative to the first magnet ring wherein ends of each magnet within the second magnet ring are located at different angular positions than ends of each magnet within the first magnet ring relative to the shaft axis of the shaft.
In another embodiment, a motor for reducing torque ripple includes a shaft having a shaft axis, a rotor positioned about the shaft, and a plurality of magnets positioned about the rotor, each magnet occupying a magnet angle xcex4 on the rotor, wherein the magnet angle xcex4 is an optimal magnet angle for minimizing cogging torque and line to line back emf harmonics.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.